Phototube and method of manufacture



May 19, 1942; R. J. CASHMAN 2,283,413

PHOTOTUBE AND METHOD OF MANUFACTURE Filed Aug. 3, 1940 INVENTOR.

Patented May 19, 1942 PHOTOTUBE AND METHOD OF MANU- FACTURE Robert J.Cashman, Evanston, 111.

Application August 3, 1940, Serial No. 350,797

18 Claims.

It is known that ultraviolet radiations having wave lengths shorter than3200 A. have the valuable effect of curing rickets and are believed tohave some prophylactic value in heliotherapy.

These radiations have also proved valuable in sterilization and incertain photo-chemical processes. of the intensity of these radiationshas, however, been difiicult if not impossible due to the lack of meanscapable of measuring only these radiations. Several forms of meters havebeen devised but these have been objectionable for various reasons, oneof the common shortcomings being the limited field from which radiationsmay be received, and another being their inability to respond only tobiologically effective radiations.

An object of my invention therefore, is to provide an apparatus and amethod of making such apparatus to accurately measure all biologicallysignificant ultraviolet radiations incident upon a horizontal plane.

Another object is to provide a method of making phototubes whichsubstantially eliminates variation in characteristics between similarlyconstructed phototubes.

Another object of my invention is to provide a phototube having anegligible response to changes in temperature.

These and other objects, which will become apparent from the followingdescription, are accomplished in general by providing a phototube havinga metal cathode disc covered on at least one face by a gas-free coatingof condensed magnesium. The phototube consists broadly of an envelopecapable of transmitting ultraviolet radiations and within which thecathode disc is rotatably mounted. The anode is preferably formed ofthin wire and is located above the cathode plate.

-In making the tube a quantity of pure magnesium is placed near thecathode plate and is associated with heating means. Initially the flatcathode plate is moved upon its supports until the surface to beactivated faces the magnesium and a small occulting disc is interposedbetween the magnesium and the cathode. Gases within the tube are removedinsofar as possibleby an efiicient vacuum pump and heat is applied tothe magnesium during the pumping process, causing a portion, orfraction, of the magnesium to vaporize and condense principally upon theocculting disc. After a portion of the magnesium has been vaporized thetube is sealed. The vaporization process is continued after sealing, asit effectively. getters or removes after the re- The accurate quantativedetermination maining active gases from the tube, which would otherwiseharmfully effect its characteristics. When the active gases have beencompletely removed by the action of the magnesium, the occulting disc iswithdrawn from its position between the magnesium and the cathode,allowing the middle fraction of the magnesium to condense upon thesurface of the cathode disc. When the desired amount of magnesium hasbeen collected the cathode disc is moved upon: 0 its supports byexternal means and secured-in its operative position. A tube formed inthis manner will. have a threshold of substantially 3200 A; that is, itwill be unresponsive to radiations having wave lengths greater thansub-* stantially 3200 A. but will respond to radiations below 3200 A.,the lower limit being determinedby the envelope material-auditsthickness. The response of my tube to such radiations is directlyproportional to their intensity, in other words the characteristics ofmy phototube are substantially linear. Because of the novel construction.of my tube practically all of the biologically effective ultravioletradiations from a single source or combination of sources incident upona horizontal plane will act upon the tube when the cathode is horizontaland the intensity of these radiations may be measured and integrated bya suitable recording device operating in conjunction with my tubeFurthermore, the response of my tube is substantially unaffected bychanges in temperature. However, it is to be understood that the basicprinciples of my method of manufacturing, hereinafter described indetail,- are applicable to phototubes formed of other materials andadapted for. use in other relations such as phototubes for. use inconjunction with television or motion picture sound apparatus.

As the threshold of a phototube is controlled 7 largely by the materialforming the cathode or cathode coating I may form a tube bymy processhaving a threshold other than 3200 A. by substituting other metals formagnesium, or if a combination of more than one metal is desired I mayemploy an alloy or mixture of two or more metals. It is not essentialthat the. material chosen have the property of adsorbing active gasesfor magnesium, or some similar material,

7 may also be vaporized in the tube without coating the cathode byproviding an auxiliary vaporizing unit.

The threshold of a tube formed by myprocess may be varied by theadmission of active gases under controlled conditions regardless of thetype of metal or mixture of metals used to form the cathode or cathodecoating. For example the threshold of 3200 A. which is characteristic ofa tube having a gas-free magnesium coating on the cathode may be raisedto 5100 A. by admitting small quantities of hydrogen and this thresholdmay be reduced to approximately 4.700 A. by re-exhausting the tube. Theadmission of gases of this nature has also been found to greatlyincrease the sensitivity of a tube.

In the accompanying drawing Fig. 1 is a perspective View of one form ofmy invention showing the relative disposition of the parts.

Fig. 2 is a sectional view of one form of mechanism employed to hold thecathode in operative position.

Fig. 3 is a perspective view illustrating another form of mechanismemployed to hold the cathode in operative position, and

Fig. 4 is a diagrammatic illustration of a circuit and recordingapparatus suitable for use in conjunction with my tube.

Referring to Fig. 1 an envelope formed of fused quartz or of glass suchas Corex D, Corning #972 or Corning 791, capable of transmittingultraviolet radiations having wave lengths of lessthan' approximately3200 A., is attached to a standard tube base It in a manner well knownin the art. Four metallic prongs, designated l3, extend through the base|2 and are connected to four' electrodes |4, I6, I! and I8. Connectionof electrode |8 to one of the prongs I3 is, however, optional and isshown as disconnected in Fig. 1 for clarity, The electrodes are disposedsubstantially parallel to the longer axis of the tube and sealed into aglass base l9 which is attached to the envelope The lower portion of theglass base I9 is preferably cylindrical in form, tapering at the top toa pinch seal or press of approximately rectangular cross-section.

All of the electrodes extend upwardly beyond the glass base, IS the twoouter electrodes I4 and I8 being attached to the under side of atransverse-metallic disc 2|. The two central electrodes l6 and I! extendthrough an opening 22 in the central portion of the transverse disc 2|,the

ends of these electrodes being spread apart after passing through thedisc 2| and connected together by a high resistance wire 23.

A- small quantity of pure magnesium 24 is placed around the resistancewire 23and is held in place by deforming the wire. A horizontalocculting disc 26 is attached to a vertical shaft L 2'! which isrotatably mounted in a vertical position by bands 36 attached to theupper portion of the electrode IT. The dimensions of the occulting disc26 are such that it may be swung 'into and out of position directly overthe magnesium 24. r r

A metal collar 3| is clamped around the cylindrical portion of the glassbase I9 by a bolt 32 and supports two upright members designated 33,.which extend upwardly parallel to the axis of the tube beyond but not incontact with the disc 2|. The upper ends of these uprights 33 are joinedtogether by a horizontal bar or axle 34 upon which two similar arms 36and 31 are journaled. A fiat metallic cathode disc 38, preferably'ofnickel, is attached to the free ends of the arms 36 and 31, the plane ofthe cathode disc being at all times parallel to the horizontal bar 34 sothat when the cathode is held in a horizontal position its plane will beat right angles to the major axis of the tube.

The anode comprises two fine curved wires, 39 extending above anddiametrically across the cathode 38 at right angles to each other. Theends of the anode wires 39 are attached to an open ring 4| which mayextend around but not in contact with the cathode disc 38. The ring 4|is attached to two diametrically opposed supports 42, the lower ends ofwhich are attached to the disc 2|. A metal prong 43 is sealed throughthe wall of the envelope I and is connected with g the collar 3| by awire 44.

Potential may be applied to the cathode 38 through the wire prong 43,the wire 44, the collar 3|, the uprights 33, the horizontal bar 34, andthe arms 36 and 31, and to the anode 39, through the electrode l4, themetal disc 2|, the supports 42, and the ring 4|. The electrode l8 servesprimarily as a support for the disc 2| and its connection with one ofthe prongs I3 is therefore optional.

As the method of manufacturing employed contemplates coating the surfaceof the cathode with condensedmagnesium vapors after sealing the envelopeand subsequently rotating the'cathode into its normal operativeposition, it is necessary to provide a latch device to hold the cathodein position after rotation. A device suitable for this purpose is shownin Fig. 2 and includes a bar 46 extending through aligned openings inthe arms 36 and 31. An L shaped member 41 is attached to the horizontalbar 34 with the plane of vertical portion of the member adjacent andparallel to the outer sideof arm 36, and an opening 45 is provided inthis portion to receive one end of the bar 46 when the cathode assumesits operative position. A coiled spring 43 acts upon the bar 46 urgingit outwardly towards member 41. One end of the vertical section ofmember 4! is bent outwardly and serves to force the bar 46 inwardlyagainst the pressure of the spring 48 when the protruding end of the bar46 is brought in contact with the vertical section of member 41 byrotation of the cathode 38. The spring 48 forces the bar 46 into theopening 45 the moment the bar and opening are aligned and so holds thecathode firmly in the desired position.

Fig. 3 illustrates another form of latch mechanism adapted to hold thecathodein operative position. Thisyform comprises a fiat spring 5|attached at its lower end to vertical member 33 and carrying an armature52 on its upper end.

A pin 53 extends through the armature 52 sub stantially parallel to thehorizontalbar 3.4, the projecting end of the pin engaging a notch in theend of a bar 54 when the cathode is in operative position. The bar 54 isattached to the arm 36 and extends laterally therefrorrn A stop,generally designated 56, is provided toprevent rotation of the cathodebeyond the desired position. andcomprises a horizontal bar 51 attachedat one end to the vertical member 33 slightly below the end of thespring 5|. The unattached end of the bar 51 is joined to a leg 58 whichextends laterally and vertically with respect to the leg 51 in a planeparallel and adjacent to the plane of the bar 54. A flat section ofmetal 59 is attached to the inner portion of the vertical leg 58. Theparts of the stop 56 are positioned in such manner that as the cathodeis rotated the arm 54 will move in a plane parallel and adjacent to theplane of .the vertical leg 58 until it comes in contact with the'upperend of the flat section 59 at which time the free end of the. arm 54will be immediately adjacent to the vertical leg 58 of the stop 56. Asmall notch BI is provided in the outer edge of the vertical leg 58positioned to register with the notch in the end of the arm 54 when thecathode has reached operative position. The pin 53 on the armature 52 isurged into engagement with these notches by the action of the spring andmay be disengaged at will by applying magnetic force to the armature 52.I prefer to use this form of holding device, as it is possible to movethe cathode into or out of operative position at will, whereas thedevice illustrated in Fig. 2 will not permit movement of the cathodeonce it has been placed in operative condition.

A phototube having the construction above described is particularlyadapted to the measure" .ment of biologically effective solarradiations, and

is intended to illustrate a novel and acceptable form of phototube forthis purpose without limiting my invention solely to the use of thistype of construction. The construction of the anode and cathode,however, imparts certain novel and highly desirable characteristics tomy tube. For example, the shadow of the anode upon the cathode isnegligible,thereby practically eliminating a source of error inmeasurements. The flat metallic cathode, in addition to broadening thefield of the phototube, also eliminates the variations found in tubeshaving curved cathodes caused by variations in the incident angle of theradiations, which affect the response of the tube.

In the following description of the process of manufacturing and in theclaims, fractional distillation is to be understood as meaning a processin which the volatilization of the metal is substantially continuous,but in which only selected portions of the vapor are used as a coatingfor the cathode, as distinguished from double distillation in which themetal is vaporized, condensed, subsequently re-vaporized andre-condensed upon the cathode. The term middle fraction is to beunderstood as meaning any'portion of metal nesium located approximatelyin the center of the resistance wire 23, and with the occulting disc 26swung into position over the magnesium. The cathode disc 38 is initiallyplaced in its inverted position with the face to be coated adjacent toand facing the occulting disc 26. The tube is then connected to anefiicient vacuum pump, not

shown, in any desired manner and is exhausted by pumping. The pumpingprocess is continued until the maximum degree of vacuum is obtained, atwhich time the vaporization process is begun by passing current throughthe electrodes 16 and H, and the resistance wire 23. The heatdevelopedby the resistance wire 23 is sufficient to cause a slow vaporization ofthe magnesium 24.

It is essential that the first fraction of magnesium vapor evolved beprevented from condensing on the surface of the cathode, as this portionof the vapor will be contaminated with active gases unavoidablyremaining in the tube. The first fraction of vapor evolved by themagsatisfactory, for the most important of these nesium is, therefore,deposited upon the occulting disc 26, the function of which is toprevent the deposition of this fractionupon the surface of the cathode.Portions of this initial fraction of magnesiumvapor will also depositupon the adjacent walls of the tube, exposing, in conjunction with theocculting disc 26, a relatively large surface to the gases within theenvelope. This initial step effectively getters or removes the activegases from within the tube asmagnesium has the property of readilytrapping such gases.

When a suitable portion of the magnesium has been vaporized the tube issealed and the vaporization is preferably continued for a few mo mentsto remove the gases formed in the tube by the sealing process. At thisstage of the procedure all active gases remaining within the now sealedtube will have been adsorbed by the magnesium condensed on the lowersurface of the occulting disc 26, and on the walls of the envelope II.

The occulting disc 26 is now swung out of its original position by asharp movement of the tube and the vapors are allowed to condense uponthe surface of the cathode 38. 'As the vaporization appears to takeplace in practically straight lines, the uppermost portion of theenvelope H will not be coated with magnesium as it is protected fromsuch deposits by the inverted cathode plate 38. When sufiicient of themagnesium has been condensed upon the surface of the oathode plate, thecurrent through the resistance wire 23 is cut off and the cathode swungaround the bar intoits operative position by a sharp movement of thetube. v

The coating so formed upon the surface of the cathode will besubstantially free of any active gases and I have found that a coatingformed in the manner described" has a photo-electric threshold ofsubstantially 3200 A, which is approximately the longest wave length ofbiologically effective ultraviolet radiations. Radiant energy strikingthe cathode, having wave lengths or more than substantially 3200 A, willnot energize the tube.

The range of responsiveness of my tube to radiations having wave lengthsof less than 3200 A, is largely determined by the thickness and materialused as an envelope. If the envelope is of thin quartz my tube willrespond to radiations having wave lengths as short as 1850 3.,or if ofCorex D, 0.3 to 0.5 mm. thick is used, it will respond to radiationshaving wave lengths as short as 2200 A. As the biologically efiectiveultraviolet radiations have minimum wave lengths of about 2000 A, anenvelope of Corex D is usually radiations have wave lengths in excess of2200 A. Filters may, of course, be used to cut out certain bands ofradiations if desired.

The effect of changes in temperature upon the characteristics of my tubeis negligible, due at least in part to the fact that the cathode surfaceis gas free, and has a negligible vapor pressure. This is in sharpcontrast to tubes formed by ordinary methods, in which changes intemperature produce marked changes in characteristics.

If I connect the above described tube to a suit-' able recording andamplifying apparatus and place the tube in an exposed position,substantially all of the biologically effective ultraviolet radiationsfrom a source, or combination of sources, such as from the sun andtheentire sky,-

will strike the fiat surface of the cathode plate 38, energizing thetube and causing the recording apparatus to operate.

To obtain a continuous record of the intensity of the biologicallyeffective ultraviolet radiations striking my tube I employ a circuitandapparatus similar to that shown diagrammatically in Fig. 4. Currentmay be supplied to the circuit through any suitable converter 62 capableof maintaining a constant voltage, and which may be any one of severalwell known types adapted to deliver 270 volts D. C. to a negativeterminal 63, and a positive terminal 64. The positive terminal 64 isconnected with the anode 39 of my phototube through a relatively highresistance 66 which may be approximately one megohm.

A condenser 61 having a capacity of approximately two microfarads isconnected to the terminals of the converter 62 through a resistance 68which controls the rate at which the condenser 6'! will be charged andwhich may be suitably of about 150,000 ohms. The condenser 61 isconnected to the cathode 69. of a relay tube generally designated 12,such as a Westinghouse trigger tube, WL 759. The coil of a recordingdevice generally designated 13 is connected in series between thecondenser 61 and the main anode H of the. tube 12. The cathode 38 of thephototube is connected to the trigger anode '14 of the tube 12 and thetrigger anodeand the cathode 69 are connected through a variablecondenser 76, this connection being grounded as shown at H.

In the above described circuit it may be seen that the condenser 51 willbe charged by the current imposed upon the terminals 63 and 64, the rateof charge being controlled by the resistance 68. Discharge of thecondenser, however, must take place between the main anode H and thecathode 69 of the tube 12, and this discharge is normally prevented bythe potential of the trigger anode 14. When radiations having wavelengths below 3200 A. strike the cathode 38 of my phototube thevariablecondenser I6 is charged by the current developed, the time requiredvarying with the setting of the condenser andthe intensity of theradiations. When the condenser 16 is charged to a certain potentialdifference the gas in the tube 72 is ionized by a discharge between thecathode 68 and the trigger anode M, which lowers the potential requiredto cause a discharge between the oathode 69 and the main anode 'H. Whenthe tube (2 assumes this condition, the condenser 61 will dischargethrough the main anode H and the cathode B9, completing the circuit. Thepassage of this current energizes the coil of the recorder 13 causingactuation of the recording device.

The intensity of the incident biologically effective ultraviolet raysmay be recorded in two ways. First, the setting of the variablecondenser 18 may be fixed anda record kept of the number of times therecorder 13 is actuated, or, if desired, the variable condenser may beprovided with a graduated scale and set to allow a fixed number ofdischarges to occur in a unit length of time, the setting of thecondenser then being a comparative measure of the intensity of theincident radiations. From the foregoing it may be seen that if desiredthe variable condenser 16 may be replaced with a fixed condenser ofpredetermined capacity, but due to the variations occurring in theintensity of the biologically effective ultraviolet rays I prefer to usethe variable condenser as shown and to record the number of times therecorder 13 is actuated within a fixed time.

The recorder I3 may be any one of several types. consists of a solenoid8| having a movable armature 82 held in its de-energized position by aspring 83. An arm 84, pivotally attached to the end of the armature 82,actuates a counter 86 which will automatically register each time thecoil 8| is energized by the passage of current through the tube 12. Thenumerals of the counter are adapted to print upon a paper tape 8!whenever a platen 88 is caused to strike the face of the counter throughthe action of an electric timing mechanism, not shown. Movement of theplaten 88 also moves the paper tape 81. The electric clock mechanism andthe counter are of standard types and need not be described in detail.It may be seen, however, that by means of this apparatus a directcomparison of the intensity of the ultraviolet rays incident upon myphototube throughout any selected period of time may be obtained.

It cannot, be stated with absolute certainty that the coating of mycathode, formed by fractional distillation or sublimation of aphoto-sensitive gas absorptive metal in a vacuum, is completely free ofactive gases, as there may be extremely small amounts of occluded gasesretained by the metal after condensation. I have determined, however,that the variable threshold commonly experienced in the manufacture ofphoto-electric tubes is largely due to gas contamination of thephoto-sensitive metal cathode coating, and that by my process sufficientquantities of these active gases are invariably removed to produce aphotosensitive material having a constant threshold, although someminute amount of gas may remain in the material. For all practicalintents and purposes my fractionally distilled photo-sensitive materialis free of active gases, and the use of the term gas free in theappended claims is to be understood as meaning gas free to the extentthat the active gases, if any, remaining in the photo-sensitive materialare present in such minute quantities as to be incapable of influencingthe photo-electric threshold of the material.

I have made the foregoing detailed description in compliance with theprovisions of section 4888 of the Revised Statutes but I do not wish tobe specifically limited to the details herein set forth, as obviouslymanymodifications may be made in the process and apparatus withoutdeparting from the true spirit and scope of my invention. While themethod of manufacture of thetube herein described has been confined tothe use of magnesium I have successfully constructed phototubes havingvarious photo-electric thresholds by the use of other metal such asbarium, strontium, calcium, titanium, tungsten, silver, copper andplatinum. Each of the tubes above mentioned is characterized by thereproducibility of characteristics. It is, of course, also possible toemploy other amplifying and recording devices in connection with myphototube.

I claim as my invention:

1. A vacuum phototube comprising an envelope having a projectingtransparent window portion, an anode and a cathode at the base of saidwindow portion, the cathode comprising a flat metallic disc rotatablewithin the envelope about an axis parallel to the disc and closelyadjacent thereto.

2. A phototube comprising an envelope having One which I have foundsuitable for use a transparent window, an anode, a cathode structureincluding a plate coated with photo-sensitive-material, means completelyenclosed by said envelope and freely rotatably supporting said cathodestructure for'free rotation of said plate towards andraway from saidwindow, and latch means engaging said cathode structure for retainingsaid plate in a position facing said window.

3. A vacuum phototube comprising a sealed envelope transparent to light,an anode, amovable cathode within the envelope, means within theenvelope for vaporizing a photo-sensitive metal, and means fordepositing any selected portion of the metal upon the cathode saiddepositing means including an occulting disc interposable between thecathode and the vaporizing means.

4. A vacuum phototube comprising a sealed envelope transparent to light,an anode, a movable cathode within the envelope, means within theenvelope for vaporizing a photo-sensitive metal, means for depositingany selected portion of the vaporized metal upon the cathode, and meansfor restraining the cathode in operative position said depositing meansincluding an occulting disc interposable between the cathode and thevaporizing means.

5. A phototube comprising a gas-impervious envelope transparent tolight, an anode, a movable cathode within the tube, a source ofphotosensitive metal within the tube, and a movable shield interposablebetween the metal vapor source and the cathode.

6. The method of manufacturing a phototube having a movable cathodecomprising the steps of degassing the metal parts, exhausting the tube,positioning an occulting disc before the cathode, vaporizing agas-adsorptive photo-sensitive metal within the tube, sealing the tubewithout discontinuing the vaporizing process, removing the occultingdisc from before the cathode and depositing the middle fraction of themetal vapors upon the cathode, and subsequently moving the cathode intooperative position.

7. A photo-electric tube comprising a shell having a transparenthemispherical window portion, a flat cathode within the shell having aphoto-sensitive surface disposed to receive radiations through thewindow portion, anode supporting means adjacent said cathode and havinno portion thereof projecting substantially above the plane of saidcathode, and a fine wire anode fixed to the anode supporting means andprojecting beyond the plane of said cathode between said photo-sensitivesurface and said window portion whereby the response of thephoto-electric tube is substantially directly proportional to the cosineof the angle of incidence of energizing radiations.

8. The method of manufacturing a photo-electric tube having a rotatablecathode including the successive steps of exposing a face of the cathodeto a photo-sensitive metal, interposing a movable shield between thecathode and the photo-sensitive metal, exhausting the tube, vaporizingthe photo-sensitive metal, sealing the tube, withdrawing the movableshield and coating the cathode with photo-sensitive metal, and rotatingthe cathode to operative position.

9. The method of manufacturing a photo-electric tube having a movablecathode and a source of vaporizable photo-sensitive metal including thesteps of shielding a surface of the cathode from the metal vapor,exhausting the tube, initiating vaporization of the photo-sensitivemetal while continuing to exhaust the tube, sealing the tube whilecontinuing vaporization of themetal and subsequently exposing saidsurface of the cathode to the metal ,vapor'whe'reby' the'metal disposedon said surface of the cathode is substantially gas free. a I V 10. Aphototube comprising a cathode, an envelope having a hemisphericaltransparent end, a cathode having a fiat photo-sensitive surface lyingsubstantially in the plane of the base of said hemispherical end,anode'supporting means having no portion abo've theplane of saidsurface, and a fine anode wire extendingfrom said supporting means topoints abovesaid surface.

11. A phototube comprising a cathode having a photo-sensitive surfacewhose periphery lies substantially in a single plane, an envelope havinga transparent portion above and enclosing said cathode, anode supportingmeans adjacent said cathode and having no portion projectingsubstantially above said plane, and fine anode wires fixed to saidsupporting means and projecting beyond said plane between saidphotosensitive surface and said window.

12. A phototube comprising an envelope having a window, pivot meansentirely within said envelope spaced from and extending across saidwindow, a flat cathode mounted to swing on said 'pivot means and topresent either face to said window, a wire anode extending between saidcathode and said window, and means for vaporizing a photo-sensitivematerial on the face of said cathode remote from said window.

13. A phototube comprising an envelope having a window, means withinsaid envelope defining an axis spaced from and extending across saidwindow, a flat cathode mounted to swing on said axis and to presenteither face to said window, a latch mounted in said envelope adjacent tosaid cathode for holding said cathode facing said window, an anode wireextending between said cathode and said window, and means for vaporizinga photo-sensitive material at a point substantially in line with thecenters of said window and said cathode and on the other side of saidcathode from said window,

14. A phototube comprising an envelope having a window, pivot means'within said envelope spaced from and extending across said window, aflat cathode mounted to swing on said pivot means and to present eitherface to said window, anode supporting means at the side of said cathode,a Wire anode fixed to said supporting means and extending between saidcathode and said window, every portion of said wire anode being farenough from said pivot means to permit free swinging of said cathode,and means for vaporizing a photo-sensitive material at a pointsubstantially in line with the centers of said window and said cathodeand on the face of said cathode remote from said window.

15. A phototube comprising an envelope having a window, means in saidenvelope opposite said window for vaporizing photo-sensitive material,an axle completely enclosed by said envelope and located between saidwindow and said means, and a cathode rotatably carried by said axle,said cathode, vaporizing means and window being in alignment wherebysaid cathode shields said window from said vaporizing means.

16. A phototube comprising an envelope having a window, a supportingstructure located in and completely enclosed by said envelope, a cathodestructure carried by said supporting structure and rotatable thereoninto a position nected to said latch mechanism for operation thereof.

18. A phototube including a gas impervious envelope containing an anodeand a movable cathode, a rigid arm on said cathode, a cathode supportadjacent to said cathode, a locking pin movably mounted on said cathodesupport and movable by magnetic force applied externally of theenvelope, and a spring operatively connected to said locking pin andnormally urging said locking pin into engagement with said rigid arm onsaid cathode.

ROBERT J. CASHMAN.

